Heat exchanger, method for operating the heat exchanger and use of the heat exchanger in an air-conditioning system

ABSTRACT

In a heat exchanger having a bank of capillary tubes, in which a fluid to be cooled and/or heated is conducted through capillary tubes and in which the capillary tubes are wetted, in concurrent flow with the fluid, by water or by a hygroscopic sorption solution, and in which air flows around said capillary tubes in countercurrent flow in relation to the fluid, the bank of capillary tubes is composed of at least one tube mat, the capillary tubes of which have a hydrophilic or water-dispersing surface with a contact angle of less than 20°.

The invention relates to a heat exchanger according to the preamble ofclaim 1, a method for operating this heat exchanger and also a use of atleast two of these heat exchangers in an air conditioner.

Capillary tubes offer good qualifications for use for example inair/water heat exchangers. They require relatively little and alsoeconomical material for production thereof and offer a relatively largeouter surface for the heat exchange and hence a heat exchanger valuewhich is higher by a multiple, for example in comparison with plate heatexchangers. In addition, they are corrosion-resistant to water andsorption solutions. Flexible plastic material tubes with an outerdiameter of 0.5 to 5 mm are termed capillary tubes.

The capillary tubes are generally combined to form mats, the tubes beingdisposed parallel to each other at a spacing of approx. 10 to 20 mm and,at the one end, are connected to a common branch for the inflow of wateror of another heating or cooling fluid and also, at the other end, to acommon branch for the return flow of the water or other heating orcooling fluid. The capillary tubes are retained in their mutual positionby spacers. Such a mat is shown for example in DE 196 40 514 A1.

A heat exchanger with a capillary tube register through which a fluid tobe cooled or heated is guided is known from EP 0901 601 B1. The tuberegister is sprinkled with water in parallel flow to the fluid andsubjected to a flow of air in counterflow to the fluid. The spacesbetween the capillary tubes are at least partially filled with foamedmaterial, as a result of which the heat exchanger surface is enlarged.One possibility for producing this heat exchanger resides in coating thecapillary tubes themselves with a foamed material coating. The foamedmaterial layer can thereby consist of the same material as the capillarytube. However, it has been shown that uniform sprinkling of the foamedmaterial layer is not possible. This is particularly true if sprinklingis effected with a sorption solution for dehumidifying the air insteadof with water. In order to obtain satisfactory efficiency of the heatexchanger, the quantity of sorption solution should be as low aspossible, if possible not more than 5% and preferably not more than 1%of the quantity of the fluid flowing through the capillary tubes. Thesevalues were however not able to be achieved for a uniform wetting of thefoamed material layer.

It is therefore the object of the present invention to indicate a heatexchanger having a capillary tube register through which a fluid to becooled or to be heated is led, the tube register being wetted with wateror a hygroscopic sorption solution in parallel flow to the fluid andsubjected to a flow of air in counterflow to the fluid, which heatexchanger has at least a higher efficiency than previous heat exchangersusing capillary tube mats.

This object is achieved according to the invention by a heat exchangerhaving the features of claim 1. Advantageous developments of this heatexchanger, a preferred method for operating this heat exchanger and alsoan expedient use of at least two heat exchangers in one air conditionerare revealed in the sub-claims.

As a result of the fact that the capillary tube register consists of atleast one tube mat, the capillary tubes of which have a hydrophilic orwater-spreading surface with a contact angle below 20°, uniform wettingof the capillary tubes takes place even with a very small quantity ofwater or sorption solution. Since the desired heat exchange is intendedto be effected between the fluid and the air, heat absorption by thenon-evaporated water or the sorption solution is disruptive since thisrepresents a heat loss. However this heat loss is all the greater, thegreater the quantity of water or of sorption solution. Therefore, thequantity ratio of water or sorption solution to fluid flowing throughthe capillary tubes should be no more than 5%, preferably no more than1%, without uniform wetting of the capillary tubes being impaired.

In order to obtain a hydrophilic or water-spreading surface, thecapillary tubes are preferably covered with a fleece. For uniformwetting, in particular a fleece made of glass fibres having a diameterof 0.1 to 0.5 mm has thereby proved favourable.

The plastic materials, such as e.g. polypropylene, from which thecapillary tubes are produced, normally have a low solid body-surfacetension and are therefore difficult to wet with water or aqueoussolutions. It results therefrom that they have no or only negligibly fewpolar groups in their structure. Therefore, in order to achieve goodwettability, they are coated advantageously with water-spreadingmaterial. Water-spreading plastic material is known for example from EP0 149 182 B2.

In order to make possible good adhesion of the water-spreading layer onthe capillary tubes, a bonding agent layer can be disposed betweenthese. This contains polar groups in sufficient quantity and isinsoluble and non-swelling in water. It can consist for example of a2.5% solution of a mixed polymer made of 87.6% by weight ofmethylmethacrylate and 12.4% by weight ofy-methacryloxypropyltrimethyoxysilane and have a thickness of 0.01 to 2μm.

The capillary tube mat can advantageously be formed from capillarylongitudinal and transverse tubes which are connected to each other inthe manner of a network for the fluid passage, at least the capillarylongitudinal tubes being connected in common by their ends respectivelyto one branch for the supply or discharge of the fluid. As a result, theheat exchanger surface can be significantly enlarged relative to the useof a mat consisting only of capillary longitudinal tubes, possibly caneven be doubled, so that the efficiency of the heat exchanger is alsocorrespondingly increased. Since the capillary transverse tubes ensurethe mutual spacing of the capillary longitudinal tubes, the spacers arealso dispensed with, it being assumed therefrom that the materialexpenditure for the capillary transverse tubes corresponds approximatelyto that for the spacers.

The configuration of the mat with capillary longitudinal and transversetubes also makes it possible to control the flow course of the fluid inthe mat by blocking the passage in individual capillary longitudinaland/or transverse tubes as desired. As a result, the mat can also beprovided with recesses both in the interior and at the edge or ameandering flow course can be adjusted in the mat. It is consequentlypossible also to configure the supply and/or discharge line for thefluid at the respective ends of the capillary tubes to be shorter thanthe corresponding side of the mat so that the flow of the air to becooled or to be heated through these is much less impeded.

The capillary tubes of the mat can be disposed such that the capillarylongitudinal and the capillary transverse tubes extend at a right anglerelative to each other. However, it is more advantageous for the flowcourse if the capillary longitudinal and transverse tubes intersect atan angle deviating from a right angle by 5° to 20°. It is particularlyadvantageous in this respect if the capillary longitudinal andtransverse tubes intersect in fact at a right angle, however areinclined respectively by 45° relative to the edges of the mat and hencerelative to the branches. In this case, both the capillary longitudinaland the capillary transverse tubes are connected directly to thebranches.

The invention is explained in more detail subsequently with reference toembodiments represented in the Figures. There are shown:

FIG. 1 a capillary tube mat having longitudinal and transverse tubeswith an inner cut-out,

FIG. 2 a capillary tube mat with an edge cut-out,

FIG. 3 a capillary tube mat with a shortened branch tube for dischargeof the fluid,

FIG. 4 a capillary tube mat with a meandering flow course,

FIG. 5 a capillary tube mat having capillary longitudinal and transversetubes extending at respectively 45° to the branches,

FIG. 6 a heat exchanger having a plurality of parallel capillary tubemats, and

FIG. 7 the schematic representation of an air conditioner.

FIG. 1 shows a capillary tube mat having capillary longitudinal tubes 1and transverse tubes 2 which intersect at a right angle, have ahydrophilic or water-spreading surface and the interiors of which areconnected to each other at the intersection points respectively suchthat a fluid flowing in the one capillary tube can enter into the othercapillary tube. The capillary longitudinal tubes 1 are connected incommon by their upper end to a branch 3 for the supply of a fluid,preferably water, and in common by their lower end to a branch 4 for thedischarge of the fluid. The fluid is hence moved in the directionindicated by the arrow 5 through the mat, said fluid flowing however notonly through the capillary longitudinal tubes but also through thecapillary transverse tubes 2. Since the capillary transverse tubes 2have the same mutual spacing as the capillary longitudinal tubes 1,their entire length is equal to that of the capillary longitudinal tubes1 and hence the surface available for heat exchange is twice as great asin the case of a mat consisting only of capillary longitudinal tubes.Correspondingly, the efficiency is also higher. The capillary transversetubes 2 also ensure that the mutual spacing of the capillarylongitudinal tubes 1 is not altered. Hence spacers can be dispensedwith.

The capillary tube mat in FIG. 1 contains an inner cut-out 6 which isfree of capillary tubes. The capillary tubes opening at the cut-out 6are configured immediately in front of these with clamps 7 so that nofluid can emerge from them but can be diverted in advance into anintersecting capillary tube.

Production of the grid-shaped capillary tube mat is relatively simple.Firstly, two half-shells are produced with respectively the contour ofhalf capillary tubes and the two half-shells are then welded together.Clamping of the capillary tubes can be effected in the case of afinished mat in such a manner that the relevant capillary tube ispressed together and the compressed inner wall is welded by heat supply.

The capillary tube mat according to FIG. 2 corresponds to that accordingto FIG. 1, however the latter is not provided with an inner cut-out butwith an edge cut-out 8.

In the case of the capillary tube mat according to FIG. 3, the lowerbranch 4 for discharge of the fluid is greatly shortened and thecapillary longitudinal tubes 1 not connected to this branch are providedwith clamps 7 at their lower end so that the fluid is diverted fromthese through the capillary transverse tubes 2 to the capillarylongitudinal tubes 1 connected to the branch 4. In order that the flowpaths for the fluid are extensively uniform, barriers 9 formed byclamping are provided furthermore in the capillary longitudinal tubes 1which are connected to the branch or abut directly so that also thefluid flowing through these passes only via a diversion to the branch 4.

The capillary tube mat according to FIG. 4 comprises two barriers 9which are obtained by clamping the capillary longitudinal tubes 1 andextend from the opposite edges of the mat respectively over half of thewidth thereof in the direction of the capillary transverse tubes 2. As aresult, the flow path of the fluid is extended in a meandering shape.This can be sensible if the fluid/air quantity ratio is small since theflow rate of the fluid should not fall below a minimum value becauseotherwise the heat exchange between fluid and air drops and the flow ofthe fluid becomes non-uniform.

In the case of the capillary tube mat shown in FIGS. 1 to 4, having afluid feed only into the capillary longitudinal tubes and havingcapillary longitudinal and transverse tubes which intersectperpendicular to each other, a diversion of the fluid by 90° is effectedat the connection points. This produces sufficient throughflow even ofthe capillary transverse tubes, this being able however to be improvedby the capillary transverse tubes extending not at a right angle but atan angle deviating from this by approximately 5° to 20°. As a result,the partial flow of the fluid passing through the capillary transversetubes can be increased, which effects an increase in heat exchangebetween fluid and air.

FIG. 5 shows a particularly advantageous configuration of the capillarytube mat. The capillary longitudinal tubes 1 and the capillarytransverse tubes 2 in fact likewise intersect each other at a rightangle, however they extend respectively at an angle of 45° relative tothe branches 3 and 4 and are also respectively connected directly tothese. The fluid hence flows out of the branch 3 directly both into thecapillary longitudinal tubes 1 and into the capillary transverse tubes 2so that these are hence supplied to the same degree and only a smallfluid exchange between them is effected. However, it is ensured that theheat exchange capacity of the capillary longitudinal tubes 1 and of thecapillary transverse tubes 2 is mutually equal, as a result of whichoptimum efficiency is achieved.

FIG. 6 shows the use of capillary tube mats, as represented for examplein FIGS. 1 to 5, in an air/water heat exchanger. The capillary tube mats10 reproduced in side view are disposed parallel to each other andvertically in one housing 11. The respective branches 3 of theindividual mats are connected to a common precursor line 12 for thewater (fluid) and the respective branches 4 of the mats 10 are connectedto a common return flow line 13. The air to be heated or to be cooled orrespectively to be humidified or to be dehumidified flows parallel tothe capillary tube mats 10 in counterflow to the water, i.e. from bottomto top, as is indicated by the arrows 14, 15, through the housing 11.

For the purposes of humidifying or dehumidifying the air, the capillarytubes of the mats 10 have a hydrophilic or water-spreading surface witha contact angle below 20°. This is supplied at as high a position aspossible of the respective mat 10, water in the case of humidifying anda sorption solution in the case of dehumidifying, which consists forexample of an aqueous lithium chloride solution. The capillary tubes ofthe mats 10 are as a result wetted over their total length as uniformlyas possible with the water or the sorption solution. For this purpose, acoating made of fleece or a water-spreading material is provided on thecapillary tubes.

Due to gravity and also due to capillary effect, the water or thesorption solution is distributed uniformly over the length of thecapillary tubes. For this purpose, the configuration of the capillarytube mat according to FIG. 5 is more suitable than that according toFIGS. 1 to 4 since all the capillary tubes are inclined to the samedegree relative to the horizontal.

When flowing down the capillary tubes of the mats 10, the sorptionsolution absorbs moisture from the counterflowing air and is conductedwith the absorbed water at the lower end of the mat 10 into a collectionreceptacle. It can then be regenerated and supplied again to the mats.The heat produced by the condensation of the moisture contained in theair is transferred by heat exchange to the water in the capillary tubesand discharged through the latter. Conversely, the heat required duringair humidification for the evaporation of the water on the capillarytubes is brought via the water flowing in the capillary tubes.

In general, it applies for air/water heat exchangers that the highestefficiency is achieved if the so-called water quantity, i.e. the ratioof temperature change of the air to the temperature change of the water,is the same over the entire surface. This requirement does not present aproblem in the case of dry cooling of air because the specific heat ofthe air, like that of the water, remains constant. In the case ofsimultaneous dehumidification of the air, the specific heat capacity ofthe air can however rise, due to the released condensation heat, to amultiple of the value of the dry air and in fact, at higher airtemperatures, greater than with lower.

If now a capillary tube mat according to FIG. 4 with a meandering fluidflow is used, then the dwell time of the fluid (water) in the region ofgreater dehumidification can be increased by a meandering formation todifferent degrees and consequently the water quantity for both media canbe kept approximately constant.

Since the degree of dehumidification can change greatly duringoperation, the meandering formation is designed for the operating pointat which high efficiency is particularly important.

FIG. 7 shows schematically an air conditioner in which two heatexchangers according to FIG. 6 are used. In the case of this airconditioner, extremely high heat recovery takes place, which makesadditional heating or cooling of the ingoing air superfluous, by a heatexchanger respectively being connected as enthalpy exchanger for theingoing air and the outgoing air.

In summer operation, the ingoing air 16 is cooled and dehumidified in afirst enthalpy exchanger 17. The cooling water flows in circulationthrough both heat exchangers. In the register of the first enthalpyexchanger 17, it is heated during cooling and dehumidification of theingoing air 16. In the register of the second enthalpy exchanger 18, thecooling water is cooled again by the outgoing air 19 after this has beencooled adiabatically to the dew point temperature thereof in a precedinghumidifier. The outgoing air 19 is consequently heated and humidifiedand subsequently discharged from the building.

In the upper part of the register of the first enthalpy exchanger 17,the coated capillary tubes are subjected to a sorption solution whichdiffuses downwards inside the coating, it being enriched with waterformed by condensation of air moisture.

In the same way, water in the upper part of the register of the secondenthalpy exchanger 18 is supplied to the coated capillary tubes, whichwater is at least partly evaporated and discharged with the outgoing air19.

1. A heat exchanger comprising: a capillary tube register, through whicha fluid to be cooled and/or to be heated is led, the capillary tuberegister configured for being wetted with water or a hygroscopicsorption solution in parallel flow to the fluid and subjected to a flowof air in counterflow to the fluid, wherein the capillary tube registercomprises at least one capillary tube mat comprising capillary tubes,the capillary tubes comprising a hydrophilic or water-spreading surfacewith a contact angle below 20°.
 2. The heat exchanger according to claim1, wherein the capillary tubes are covered with a fleece.
 3. The heatexchanger according to claim 2, wherein the fleece comprises glassfibres having a diameter of 0.1 to 0.5 mm.
 4. The heat exchangeraccording to claim 1, wherein the capillary tubes are coated with alayer comprising water-spreading material.
 5. The heat exchangeraccording to claim 4, comprising a bonding agent layer between thecapillary tubes and the layer comprising water-spreading material. 6.The heat exchanger according to claim 1, wherein the capillary tube matcomprises capillary longitudinal and transverse tubes which areconnected to each other in the manner of a network for the fluidpassage, at least the capillary longitudinal tubes being connected incommon by their ends respectively to one branch for the supply anddischarge of the fluid.
 7. The heat exchanger according to claim 6,configured, for control of the flow course of the fluid in the capillarytube mat, wherein the passage for the fluid is blocked in individualcapillary longitudinal and/or transverse tubes.
 8. The heat exchangeraccording to claim 7, wherein at least one branch of the capillary tubemat is shorter than a length of a side of the capillary tube matparallel thereto.
 9. The heat exchanger according to claim 7, whereinthe capillary tube mat comprises cut-outs in the interior or at theedge.
 10. The heat exchanger according to claim 7, wherein a flow coursein the capillary tube mat is meandering.
 11. The heat exchangeraccording to claim 10, wherein a degree of meandering of the flow courseinside the capillary tube mat changes.
 12. (Canceled)
 13. The heatexchanger according to claim 6, wherein the capillary longitudinal tubesand the capillary transverse tubes respectively extend diagonallyrelative to the branches.
 14. The heat exchanger according to claim 6,wherein the capillary longitudinal tubes and the capillary transversetubes extend respectively at an angle of 45° relative to the branchesfor the supply and discharge of the fluid and are connected directly tothese.
 15. The heat exchanger according to claim 1, wherein thecapillary tube register comprises a plurality of capillary tube matswhich are disposed parallel to each other, having a common supply linefor the fluid on a side and a common discharge line for the fluid on anoppositely situated side.
 16. A method for operating a heat exchanger,the method comprising: altering a humidity of air using a capillary tuberegister, through which a fluid to be cooled and/or to be heated is led,the capillary tube register configured for being wetted with water or ahygroscopic sorption solution in parallel flow to the fluid andsubjected to a flow of air in counterflow to the fluid, wherein thecapillary tube register comprises at least one capillary tube matcomprising capillary tubes, the capillary tubes comprising a hydrophilicor water-spreading surface with a contact angle below 20°; wherein thealtering the humidity of air comprises dehumidifying air or humidifyingair, wherein the dehumidifying comprises wetting the surface of thecapillary tubes uniformly with a hygroroscopic sorption solution anddischarging condensation heat of moisture withdrawn from the air usingthe fluid which is lead through the capillary tube register, and whereinthe humidifying comprises wetting the surface of the capillary tubesuniformly with water and delivering evaporation heat for humidifyingusing the fluid which is lead through the capillary tube register. 17.The method according to claim 16, wherein the altering the humiditycomprises dehumidifying and wherein the sorption solution is an aqueouslithium chloride solution.
 18. The method according to claim 16 whereinthe altering the humidity comprises humidifying air, including wettingthe surface of the capillary tubes uniformly with water and deliveringthe evaporation heat for humidifying the air using the fluid which isled through the capillary tube register.
 19. The method of claim 16,comprising using at least two of the heat exchangers in an airconditioner, including subjecting the heat exchangers to a flow of thefluid in a closed circulation in succession, the first heat exchangerbeing used for cooling and dehumidifying the ingoing air and the secondheat exchanger for cooling the fluid by the outgoing air.
 20. The methodaccording to claim 19, comprising cooling the outgoing air adiabaticallyto the dew point temperature thereof before flowing through the secondheat exchanger.
 21. The method according to claim 19, comprising wettingthe surface of the capillary tubes of the first heat exchanger withsorption solution and that of the second heat exchanger with water.